(1) Field of the Invention
This invention relates to a radiation shielding material suitable for simultaneously intercepting .alpha., .beta., .gamma. and x rays as well as neutrons and also relates to a process for preparing the radiation shielding material.
(2) Prior Art
In a process for intercepting (shielding from) radiation, radiation to be taken into account may mainly be .gamma. rays, x rays and neutrons. In a case where .alpha. rays and .beta. rays are accompanied by .gamma. rays and x rays, a shielding material which is capable of intercepting .gamma. rays, x rays and neutrons will of course be also capable of intercepting .alpha. rays and .beta. rays.
Thus, massive substances, such as concrete, lead plates, lead blocks and iron plates, have heretofore been used as construction materials for intercepting (shielding from) .gamma. rays and x rays.
Neutrons have a very wide variety of energy, are therefore complicated and show reactions such as elastic scattering, non-elastic scattering and neutron capture. Thus, water, paraffin, polyethylene and the like which have a high hydrogen content, are used as a retarder for neutron velocity generated by elastic scattering, while concrete and lead which are massive are used because of the emission of .gamma. rays in case of non-elastic scattering. In addition, since highly penetrable .gamma. rays are generated by the emission of other particles and photons due to neutron capture, it is necessary to use a massive substance in order to intercept such .gamma. rays. More particularly, since neutrons hardly constitute a single source of radiation and, in many cases, they are accompanied with .gamma. rays, interception of .gamma. rays must also be taken into account in case of interception of neutrons. Accordingly, a radiation shield is composed not only of a shielding component such as concrete, iron, lead or the like but also of another shielding component such as water, paraffin, polyethylene or the like.
Further, in an installation wherein radiation is handled, a concrete construction absorbs water contaminated with radioactivity because of its water absorbability, and it is very difficult to wash and clean the thus contaminated concrete construction. Thus, the concrete construction has actually been coated on the surface with a water-proof paint or a polymer thereby to enhance the concrete in hydrophobicity, but the concrete construction so coated is very expensive. Moreover, it is necessary that a large quantity of water be completely controlled without its leakage and that combustible materials, such as paraffin and polyethylene, be thoroughly controlled from the viewpoint of fire prevention.
It is necessary that the aforesaid concrete or iron construction have both a strength required as such and a considerable wall thickness in proportion to the dose of radiation; thus, the construction comes to be a very heavy structure. Lead is an excellent shielding material, but it is, per se, unsuitable for use as a material for a construction because of being a soft material and is therefore required to be used with a concrete or iron construction with the result of a high construction cost being needed. Further, it needs a special water tank, paraffin-made container and the like to intercept neutrons. In this case, particularly the water tank should be given close attention and brought under control to absolutely eliminate leakage from the water tank, and it is thought desirable that the water used be almost pure water containing none of heavy metal ions and the like. It is also necessary to pay close attention particularly to heat resistance and fire prevention since paraffin and polyethylene are apt to melt and ignite at comparatively low temperatures.
Accordingly, it needs a very complicated and expensive installation to intercept .gamma. rays, x rays and neutrons at the same time. In addition, it is necessary that concrete be waterproofed in a special way because of its high water absorbability.